Europa's Thermal Emission
The Atacama Large Millimeter Array (ALMA) allows us to see Europa's thermal emission from Earth. I have been using ALMA to investigate the nature of Europa's surface thermal structure, constrain its surface thermal properties, and search for thermal anomalies potentially indicative of geologic activity.
A Hotspot on Europa?
Europa is an ocean world, and its young, fractured surface suggests a history of geologic activity. Any sites of ongoing activity, where relatively warm subsurface material is transported to or near the surface could manifest as hotspots—locations with elevated surface temperatures relative to their surroundings. I used ALMA data of thermal emission from the surface to investigate one such potential hotspot: a location near Pwyll crater that exhibited anomalously high nighttime temperatures in Galileo thermal data and that is associated with two potential plume detections. The same location, it turns out, is actually anomalously cold in our ALMA data, suggesting that its nighttime temperature is better explained by a locally elevated thermal inertia than by an endogenic hotspot. For details, please see the paper. For a more comprehensive, but general explanation, check out this blog post.
Figure adapted from Trumbo et al. (2017). Both the Galileo and ALMA data points can be fit by assuming an anomalously high thermal inertia near Pwyll crater, rather than an active hotspot.
ALMA observations of Europa compared to the global thermal from Trumbo et al. (2018).
Surface Thermophysical Properties
Using ALMA, we obtained four thermal observations of Europa, which comprise the first complete map of its thermal emission. To provide a comparative baseline, I developed a simple thermal model to simulate the ALMA observations. This model calculates the emission from Europa's surface at ALMA wavelengths using spatially varying bolometric albedos and assuming passive absorption of sunlight and greybody emission to space. The resultant brightness temperatures are dependent on the thermal inertia and millimeter emissivity of the surface, which are taken as free parameters in the modeling. The figure at left shows a comparison between the data and model images, assuming a globally homogenous emissivity and thermal inertia. The resemblance is striking, suggesting that the dominant control on Europa's daytime temperature patterns is spatial variation in its albedo. However, there are a number of localized differences between the data and model images. One way to visualize these differences is to make maps of the thermal properties that can explain them (thermal inertia and millimeter emissivity, in this case). We can then search for any correlation between the potential surface thermal properties and the geology or surface composition of the surface. In this paper we do just that, and, while we do not identify any obvious large-scale correlations, we do identify one puzzling cold spot. See a nice EOS writeup of this finding here, and check out the ALMA press release here!
Thermal Anomalies
Our analysis of the initial ALMA images of Europa revealed numerous spatially localized thermal anomalies (both hot and cold), which might reflect spatial heterogeneity in the surface thermal properties or, in the case of anomalously warm regions, active geology. But, telling the difference requires observing locations of interest at multiple times of the Europa day, as different underlying causes
of thermal anomalies predict different behavior throughout the diurnal cycle. We recently obtained many additional ALMA observations, which provide the needed diurnal coverage. Using my thermal model (see right), we can simulate the expected temperatures for each image under various scenarios to determine the origins of these anomalies and constrain active heating across the surface. Stay tuned!
Animation showing Europa's modeled dayside thermal emission as it rotates.